Compression test and Octane Requirement

[hardflex]

If you measure the compression in your motor and they are equal, can you also use that number to determine what octane gas is required for it? In other words, for example, each cyl measures 120lbs, so it needs 89 octane. 150lbs plus needs 92 or something like that. It would seem to be a strong corellation between them.

Mine is a 2.0 now 2056 with the flat top pistons, supposed to be about 8.0 compression. I'm told 89 octane is OK. How low does compression need to go to be able to burn 87 safely?

[mattr_914]

Compression ratio determines octane. A really high compression ratio will need a slower burn (higher octane) to prevent detonation. The compression read with a gauge has a correlation, but its not accurate enough, in my opinion, to base your octane off. If you have an 8:1 cr, which seems really low for flat top pistons, I would say you are fine on 87 gas. I put KB pistons in my 2056 and set the compression ratio to 9.2:1 by trimming the pistons, but to calculate that you have to know the volume in the head, deck height, bore, and stroke which is all easy if your motor is apart.

[hardflex]

semi dished pistons which were stock in the 2.0 914's i think were 7.6-1. My heads are stock 2.0 as well. seems i've seen the 8.0 number here at times, that's where it comes from.

I filled up w gas this morning, premium, just cause I don't want preignition to fry my motor. I can't hear the knocking as easily as in some "quieter" motors, so I tend to spring for the extra dollars. But I really was inquiring because I have other cars I wanted to apply the logic to as well.

[Joe Ricard]

The compression of 150 pounds is totally not related to te compression ratio.

if you have a 2056 with 8 to 1 compression 89 octane will be just fine. actually make more power as lower octane fuel is more combustable.

[Dave at Pelican Parts]

Different octane grades of fuel do not necessarily have more or less energy than others. That is down to the exact formulation of each one, and that varies. And I think it's probably down in the noise anyway.

Your measured compression pressure is only indirectly related to your compression ratio. In turn, the latter has a large effect on what grade of fuel you need to run, but there are many other factors that influence it.

For example, if you have a very wild cam much of the compression will 'bleed out" when the intake and exhaust valves are open at "overlap". That reduces the octane that engine needs, and reduces the compression numbers. Conversely, sharp peaks in the combustion chamber will tend to make the mixture spontaneously ignite more easily, as they provide hot spots for ignition to start. Those are completely unrelated to compression pressures and compression ratios, but they will mean your engine will require higher octane fuel.

Those are two very simple examples, but lots of other factors seem to go into what grade of fuel an engine needs. The only real way to tell on your motor is to try different grades and see how they work...

--DD